This invention relates generally to a process for preparing alkali metal chlorates, and more particularly, to an improved method for making various forms of inorganic chlorate, such as sodium chlorate crystals and sodium chlorate-containing solutions especially suitable for use in processes and devices for generating chlorine dioxide.
Alkali metal chlorate, and in particular sodium chlorate has been produced by the electrolysis of aqueous solutions of alkali metal chlorides, such as sodium chloride in electrolytic cells equipped with or without membranes or diaphragms. Typically, electrolytic cells make chlorates within the cell by reacting chlorine produced at the anode with alkali metal hydroxide produced at the cathode. One such representative electrolytic cell of this type is shown in U.S. Pat. No. 3,732,153 by C. J. Harke et al. Various other arrangements of both electrochemical and combinations of electrochemical and chemical methods for manufacturing chlorates have also been proposed, such as the use of a two compartment permselective membrane equipped electrolytic cell operating in conjunction with a diaphragmless-type electrolytic chlorate cell. This method is disclosed in U.S. Pat. No. 3,897,320 to E. H. Cook. However, to obtain improved current efficiencies and significant reductions in electrical power requirements in the production of inorganic chlorate, U.S. Pat. 3,464,901 provides for the electrochemical preparation of chlorine and caustic soda in a diaphragm type chloralkali cell. The caustic soda containing unreacted alkali metal chloride and alkali metal chlorate is then removed from the cell and mixed and chemically reacted with chlorine from the anolyte of the cell. The chemical reaction is carried out at a pH of 6 to 8 to convert the alkali metal hypochlorite to chlorate. However, in order to maintain the conditions most favorable for converting hypochlorite to chlorate additional caustic and/or acid over and above that supplied by the cell has to be added to the reaction mixture. In the case of Japanese Pat. No. 792,025 dilute chlorine is reacted with less than 20 percent caustic soda to produce a concentrated sodium hypochlorite solution with sufficient caustic remaining in it to produce a pH of 8 to 10. The solution is subsequently diluted from about 13 to 15 percent sodium hypochlorite to 6 to 8 percent sodium hypochlorite with recycled stream of alkali metal chloride and chlorate. The diluted stream is then acidified with hydrochloric acid to a pH of about 6.0 and finally fed to an electrolysis cell.
It has now been discovered that in the process of chemically preparing an alkali metal chloride/chlorate-containing feedstock for the electrochemical preparation of alkali metal chlorate, the steps of dilution and acidification to achieve conditions most favorable for converting alkali metal hypochlorite to chlorate in the reaction mixture can be eliminated. The improved process of the present invention maximizes the conversion of alkali metal hypochlorite to chlorate without additional acid being added over and above that supplied as initial reactant in order to control or lower the pH during the mixing, reacting and ageing of the reaction mixture.
Accordingly, it is the principle object of the present invention to provide an improved continuous process for the combined chemical and electrochemical preparation of alkali metal chlorates.
It is a further object of the instant invention to provide a process which will maximize the chemical conversion of alkali metal hypochlorite to alkali metal chlorate without further acidification of the reaction mixture.
A still further object of the present invention is to prepare chemically a near neutral alkali metal hypochlorite/chlorate/ chloride solution requiring no further dilution with brine etc. or pH adjustment as the sole source of electrolyte feed for the electrolytic production of alkali metal chlorates.
These and other objects, features and advantages will become apparent to those skilled in the art after a reading of the following more detailed description.